As silicon-based circuits achieve faster operating speeds chip-to-chip delays become a limiting factor in high speed computer applications. To overcome this problem optical interchip communication has been suggested as a means to limit the delay between chips. However, silicon is not an active optical material. Thus, optical sources disposed on a silicon integrated circuit require either a hybrid-material or a hybrid packaging technology to be fabricated. For example, for complete integration GaAs/Si and other hybrid structures must be fabricated. In such hybrid-structure technology high speed GaAs circuitry may be employed to modulate an optical signal being transferred through a silicon integrated circuit either by directly modulating the light source, or by taking advantage of the electro-optical activity of the material to modulate the light itself. An alternative hybrid packaging approach where, for example, the GaAs light source is grown on a GaAs substrate and driven by silicon circuitry from another chip has limitations in terms of modulation speed and flexibility. One limitation arises in those situations wherein the light source is split into many channels that need to be independently modulated.
In general, such hybrid materials exhibit fundamental material and processing problems. As a result, it is a desirable goal to provide on-chip modulation of a dc driven optical radiation source with a silicon-based integrated circuit without requiring the provision of a hybrid-material optical modulator. It is also a desirable goal to provide an optical modulator that is responsive to electrical signals, that is an opto-electronic modulator, such that light is modulated in accordance with electrical signals generated on the chip.
In U.S. Pat. No. 4,745,449, issued May 17, 1988 Chang et al. disclose a Group III-V FET-type photodetector. In U.S. Pat. No. 4,517,581, issued May 14, 1985, Thompson discloses a photodetector in the form of a JFET in which the gate is defined as a rib that also functions as an optically absorbing optical waveguide. In U.S. Pat. No. 4,360,246, issued Nov. 23, 1982 Figueroa et al. disclose an integrated waveguide and FET detector. The devices of both Thompson and Figueroa et al. are constructed of Group III-V material.
In U.S. Pat. No. 4,729,618, issued Mar. 8, 1988 Yoshida et al. disclose a hybrid integrated optical circuit that provides a high cost and high performance substrate material such as InP or GaAs for a portion of the circuit while other portions having a relatively low degree of integration, such as a bent waveguide, are formed of a low cost substrate material.
In U.S. Pat. No. 4,438,447 issued Mar. 20, 1984 Copeland, III et al. disclose an electro-optic integrated circuit having on-chip electrical connections that are replaced by an optical waveguide layer. The semiconductor chip of Copeland is comprised of Group III-V material.
In JP55-138889 Mita discloses a semiconductor pulse generator said to have an integrally formed light oscillator and a FET through a light waveguide on a monocrystalline substrate.
In a journal article entitled "Guided wave GaAs/AlGaAs FET optical modulator based on free carrier induced bleaching", Electron. Lett. (UK), Vol. 23, No. 24, 19 Nov. 1987, pp. 1302-1304 Abeles et al. disclose a Group III-V optical modulator based on a free carrier bleaching effect. The device is said to be a single quantum well FET optical modulator having a FET gate self aligned to a waveguide. A similar device is also disclosed by Abeles et al. in a journal article "Novel Single Quantum Well Optoelectronic Devices Based on Exiton Bleaching", Journal of Lightwave Technology, Vol. LT-5, No. 9, September, 1987.
A. Alping et al. disclose reverse-biased ridge waveguided AlGaAs double heterostructures in a journal article entitled "Highly efficient waveguide phase modulator for integrated optoelectronics", Appl. Phys. Lett., Vol. 48, No. 19, 12 May 1986.
However, this prior art does not disclose an optical modulator for silicon-based integrated circuits.
In an article entitled "Integrated All-Optical Modulator and Logic Gate for Fiber Optics Systems", IGWO 1988, pps. 351-354, R. Normandin et al. disclose a silicon-based modulator for radiation within the range of 1.32 to 1.55 microns. Modulating light is absorbed in the waveguide and creates electron-hole pairs via indirect interband absorption, thereby effectively lowering the waveguide's index of refraction at the wavelength of the guided light. The modulator of Normandin is thus controlled by an optical signal and not by an electrical signal.
In an article entitled "Infra-red Light Modulator of Ridge-Type Optical Waveguide Structure Using Effect of Free-Carrier Absorption", Electronics Letters, 14/8/86, Vol. 22, No. 17, pp. 922-923, S. Kaneda et al. disclose the optical modulation of 10.6 micron radiation with a ridge waveguide structure including a p+ Si substrate (carrier concentration 5.times.10.sup.18 /cm.sup.-3) and a p- Si waveguide (carrier concentration 1.times.10.sup.14 /cm.sup.-3) overlying the substrate (FIG. 4). A tunnel MIS diode functions as an electrode for carrier injection.
In an article entitled "All-Silicon Active and Passive Guided-Wave Components for .lambda.=1.3 and 1.6 .mu.m", Journal of Quantum Electronics, Vol. QE-22, No. 6, June 1986, R. A. Soref et al. disclose end coupled planar and channel waveguides at 1.3 microns fabricated in single-crystal Si layers grown epitaxially on heavily doped Si substrates.
FIG. 1a shows in cross-section a ridge waveguide device 10 similar to that disclosed by Soref et al. The device 10 includes an n+&lt;111&gt; Si substrate 12 that is doped at a concentration of 3.times.10.sup.19 cm.sup.-3. Overlying the substrate 12 is an n-type epitaxial layer Si layer 14 having a ridge structure 16. The layer 14 is doped at a concentration of 9.times.10.sup.14 cm.sup.-3. Guided Light (GL) occupies a channel-waveguide region of the device 10. Due to the relatively small difference between the index of refraction of the layers 14 and 12 the layers are required to have a considerable thickness. By example, the dimensions of the device of FIG. 1a are A=10 microns, B=2.8 microns and C=4.2 microns. As a result this type of device has dimensions that are less than optimum for providing structure compatible with silicon based VLSI integration rules.
In a journal article entitled "Light-by-Light Modulation in Silicon-on-Insulator Waveguides", IGWO 1989, pps. 86-87 R. A. Soref et al disclose the optical modulation of 1.3 micron light with 800 nm light directed onto a ridge waveguide structure. FIG. 1b illustrates in cross-section a similar modulator 20. An n-type Si substrate 22 has a 0.4 micron thick (D) buried oxide layer 24 formed over a surface thereof. Overlying the oxide layer 24 is a 0.15 micron thick crystalline silicon layer over which a 3.0 micron thick epitaxially-grown Si layer is formed (C+B). Into the Si epilayer were etched strip waveguides having widths (A) of 5, 10, 20 or 40 microns. This device was said to be the first Si-on-SiO.sub.2 ridge guide reported in the literature. However, this device operates by optical and not optic electronic control.
It is thus one object of the invention to provide an opto-electronic modulator for a silicon-based integrated device, the opto-electronic modulator including a silicon waveguide region and an adjacent electrical insulator region.
It is another object of the invention to provide a SOI opto-electronic modulator having dimensions amenable to fabrication with VLSI circuits.
It is another object of the invention to provide an opto-electronic modulator/waveguide configuration wherein the modulation is performed by a change in an index of refraction caused by free carrier injection.
It is another object of the invention to provide a silicon-based integrated circuit having an on-chip opto-electronic modulator that does not require hybrid-material processing technologies.